In the prior art, there is known a writing instrument having a lower barrel having mounted therein two colored ball point pen units and an upper barrel provided with a cam mechanism engaging with said units. By turning said upper barrel through a certain angle around the axis of the barrel, said units can be selectively shifted into a writing position or retracted position.
However, in said cam mechanism of the writing instrument, the upper barrel can not be shifted in the axial direction, so that even if one of said ball point pen units is replaced by a mechanical pencil unit, the feeding out of the lead can not be effected by axial shifting of the upper barrel.